The index profile of optical fibers is generally described by the shape of the graph of the function that associates the radius and the refractive index of the fiber. It is conventional to plot the distance r from the center of the fiber along the abscissa axis and the difference between the refractive index and that of the cladding of the fiber up the ordinate axis. Thus the terms “step”, “trapezium” and “triangle” index profile are used for graphs which are respectively step-shaped, trapezium-shaped and triangular. These curves generally represent the theoretical or set point profile of the fiber, and fiber fabrication constraints can yield a significantly different profile.
It is advantageous to manage chromatic dispersion in new high bit rate wavelength division multiplexed transmission networks, especially for bit rates of 40 Gbit/s or 160 Gbit/s and above, the objective is to obtain substantially zero cumulative chromatic dispersion over the link for all the wavelength values of the multiplex, so as to limit widening of the pulses, the expression “cumulative chromatic dispersion” refers to the integral of the chromatic dispersion over the length of the fiber, for constant chromatic dispersion, the cumulative chromatic dispersion is equal to the product of the chromatic dispersion multiplied by the length of the fiber. A cumulative dispersion value of a few tens of picoseconds per nanometer (ps/nm) is generally acceptable. In the vicinity of the wavelengths used in the system it is also beneficial to avoid chromatic dispersion having zero values locally, since non-linear effects are greater at zero values. Finally, it is also beneficial to limit the cumulative chromatic dispersion slope over the range of the multiplex in order to prevent or limit distortion between the channels of the multiplex. The chromatic dispersion slope is the derivative of chromatic dispersion with respect to wavelength.
In optical fiber transmission systems it is conventional for line fibers to be stepped index fibers, also known as single mode fibers (SMF), or to be non-zero dispersion shifted fibers (NZ-DSF+). The term NZ-DSF+ is used for shifted dispersion fibers having positive non-zero chromatic dispersion at the wavelengths at which they are used, which are typically around 1550 nm. At these wavelengths the fibers have low chromatic dispersion, typically less than 11 picoseconds per nanometer kilometer (ps/(nm.km)) and a chromatic dispersion slope from 0.04 ps/(nm2.km) to 0.1 ps/(nm2.km) at 1550 nm.
Using short lengths of dispersion compensating fiber (DCF) to compensate chromatic dispersion and chromatic dispersion slope in SMF or NZ-DSF+ line fiber is known in the art. DCFs are described in various patents. In the vicinity of a wavelength of 1550 nm they have negative chromatic dispersion, which compensates the cumulative chromatic dispersion in the line fiber, and can also have negative chromatic dispersion slope, which compensates the positive chromatic dispersion slope of the line fiber. U.S. Pat. No. 5,568,583 and U.S. Pat. No. 5,361,319 propose a dispersion compensating fiber for compensating the chromatic dispersion of a single mode fiber with dispersion of the order of 17 ps/(nm.km) at 1550 nm. FR-A-99 08 298 and FR-A-01 06 246 describe dispersion compensating fibers adapted to compensate the chromatic dispersion and the chromatic dispersion slope of a dispersion shifted fiber. The cost of dispersion compensating fiber is generally higher than that of line fiber, and dispersion compensating fiber also has high attenuation.
One problem encountered in transmission systems is that of compensating chromatic dispersion over the whole of the bandwidth, in this context, a plurality of bands is generally defined arbitrarily within the range of wavelengths that can be used for optical fiber transmission, the C band extends from 1530 nm to 1565 nm and the L band is above the C band and extends up to wavelengths of the order of 1620 nm to 1625 nm. The XL or U band is above the L band and comprises wavelengths from 1625 nm to 1675 nm.
Prior art chromatic dispersion compensating fibers are generally difficult to use for compensating chromatic dispersion and chromatic dispersion slope in the U band. They are usually optimized for an operating point around 1550 nm, accordingly, at a wavelength of 1650 nm, they do not compensate chromatic dispersion or chromatic dispersion slope well, and they also have high bending losses in the U band.